Poly-steel double wall tank

Abstract

An improved double wall tank has two or more threaded fittings, with a steel outer tank and a thermoplastic inner tank. The inner tank is formed within the outer tank by rotational molding. The inner tank is bonded to the outer tank in an area of the fittings, but is otherwise separate and apart from the outer tank. Before carrying out the rotational molding process, bonding material is applied to the outer tank in the area of the fittings. The thermoplastic material is in powder form and includes mold release material. The thermoplastic material is inserted into the outer tank before rotational molding. The degree of shrinkage of the inner tank can be controlled by air pressure including an area between the fittings.

I claim:  
     
       1. A double wall tank comprising an outer tank of steel and an inner tank of thermoplastic material, said outer tank having an inner surface comprising first areas and a second area, said first areas being located adjacent to each of at least two fittings that are separate and apart from one another, said first areas each including an interior surface of said tank located beyond said fittings and surrounding said fittings, said second area being a remainder of said inner surface that is not said first areas, the thermoplastic material being bonded directly, without clamping, to an inner surface of said outer tank in said first areas and being not bonded to said outer tank in said second area. 
     
     
       2. A tank as claimed in  claim 1  wherein the thermoplastic material of said inner tank is selected from the group of polypropylene, nylon and polyethylene. 
     
     
       3. A tank as claimed in  claim 1  wherein an opening of each of said fittings is closed by a cover to which said thermoplastic material is bonded. 
     
     
       4. A tank as claimed in  claim 1  wherein said fittings are threaded fittings. 
     
     
       5. A tank as claimed in  claim 1  wherein there is a vacuum for monitoring an interstice between said inner tank and said outer tank for leaks. 
     
     
       6. A tank as claimed in  claim 1  wherein said inner tank is substantially of uniform thickness. 
     
     
       7. A tank as claimed in  claim 1  wherein the fittings are located along one side of the tank. 
     
     
       8. A tank as claimed in  claim 1  wherein the fittings are not lined with thermoplastic material. 
     
     
       9. A tank as claimed in  claim 1  wherein the tank has flanged fittings and said flanged fittings are lined with thermoplastic material. 
     
     
       10. A tank as claimed in  claim 1  wherein the inner tank is self supporting, except for being bonded at the fittings to the outer tank.